Yemeni security forces arrested on Saturday a woman thought to be involved in sending explosive packages headed to the United States after surrounding a house where she was hiding in the capital Sanaa, a security official said.

Yemen's President Ali Abdullah Saleh said earlier that security forces had surrounded a house at an undisclosed location where a woman believed to be involved in the apparent plan to attack targets in the United States was taking refuge.

The bombs had hallmarks of the militant group al Qaida or its affiliate al Qaida in the Arabian Peninsula, U.S. Homeland Security Secretary Janet Napolitano said on Saturday.

On Friday, U.S. President Barack Obama said at the White House that packages containing explosives found in Britain and Dubai had been addressed to two synagogues in Chicago.

Napolitano told CNN on Saturday that the parcel bombs appeared to include the same explosives used in the Christmas Day attempted attack, pentaerythritol trinitrate, or PETN.

British Home Secretary Theresa May said Britain would act immediately to stop the movement of all unaccompanied air freight from Yemen into or through the United Kingdom.

Direct cargo and passenger flights from Yemen to Britain were suspended in January, following an attempt to bomb an aircraft destined for Detroit, but May said more precautionary measures were needed.

The device found on a U.S.-bound cargo plane at a British regional airport was "viable" and could have brought down an aircraft if it had exploded, May said on Saturday.

Yemeni authorities are checking dozens more packages in a search of the terrorists who tried to mail bombs to Chicago-area synagogues in a brazen plot that heightened fears of a new al-Qaida terror attack.

The plot sent tremors throughout the U.S., where after a frenzied day searching planes and parcel trucks for other explosives, officials temporarily banned all new cargo from Yemen.

The Obama administration has been increasingly focused on the al Qaida affiliate, which authorities have said was behind the failed attempt to blow up a U.S. jetliner on Christmas Day last year with a bomb that a Nigerian man hid in his underwear.

But Napolitano told NBC News that the investigation was continuing and that it was too early to say whether the packages were meant to detonate while airborne on the massive cargo planes or when they reached their destinations, Jewish centers in Chicago.

Obama called the coordinated attacks a "credible terrorist threat." The bombs were discovered just days before the U.S. mid-term national elections, in which the discussion of terrorism has been almost completely absent.

A Yemeni security official said investigators there were examining 24 other suspect packages in the capital, San'a. Authorities were questioning cargo workers at the airport as well as employees of the local shipping companies contracted to work with FedEx and UPS, the official said.

The first of the two bomb parcels was found in Dubai in a FedEx shipment that originated in Yemen and contained white powder explosives within the ink cartridge of a computer printer, said a Yemeni police statement. The device was rigged to an electric circuit, and a mobile phone chip was hidden inside the printer, the statement said.

The U.S. has FBI, military and intelligence officers stationed in Yemen to conduct an inquiry. There are only a handful of international shipping locations in the impoverished Arab nation, but U.S. officials worried that record keeping would be sparse and investigators would have to rely more on intelligence sources to identify the would-be bombers.

In San'a, there was no visible security presence Saturday at the UPS and FedEx offices, which are located on the same street. An employee at the UPS office said they had been instructed not to receive any packages for delivery for the time being.

The terrorist efforts underscore the necessity of remaining vigilant against terrorism, U.S. President Barack Obama said in the hours after officials disclosed the interception of the explosive devices.

The U.S. Homeland Security Department said it was stepping up airline security, but White House spokesman Robert Gibbs said Americans did not need to change their travel plans. May also said that the U.K. does not intend to change the country's "threat level" at this stage.

Intelligence officials were onto the suspected plot for days, officials said. The packages in England and Dubai were discovered after Saudi Arabian intelligence picked up information related to Yemen and passed it on to the U.S., two officials said.

U.S. intelligence officials warned last month that terrorists hoped to mail chemical and biological materials as part of an attack on the United States and other Western countries.

Since the failed Christmas bombing, Yemen has been a focus for U.S. counterterrorism officials. Before that attack, the U.S. regarded al-Qaida's branch there as primarily a threat in the region, not to the United States.

The Yemen branch, known as al-Qaida in the Arabian Peninsula, has since become a leading source of terrorist propaganda and recruiting. Authorities believe about 300 al-Qaida members operate in Yemen.

Those numbers would include the radical U.S.-born Muslim cleric Anwar al-Awlaki, who now is in hiding in Yemen. He has been linked in the Christmas attack and has inspired other terrorists with his messages advocating violence. Also hiding in Yemen is Samir Khan, an American who declared himself a traitor and helps produce al-Qaida propaganda.

The Yemeni government has stepped up counterterrorism operations, with help from the U.S. military and intelligence officials.